Everything except quota and squirrelmail.
I have a lot(1-3 per day) of false-positives in my spam quarantine folder, and 90% of those are because of bad header. (most bad header senders uses Outlook, no suprises there.)

I dont want to quarantine bad header mails at all, i want them delivered to the recipients mail box!
It would be ok to tag bad header mails in the subject, but definetley no quarantine.